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Abstract

Although confocal infrared (IR) absorption micro-spectroscopy is well established for far-field chemical imaging, its scope remains restricted since diffraction limits the spatial resolution to values a little above half the radiation wavelength. Yet, the successful implementations of below-the-diffraction limit far-field fluorescence microscopies using saturated irradiation patterns for example for stimulated-emission depletion and saturated structured-illumination suggest the possibility of using a similar optical patterning strategy for infrared absorption mapping at high resolution. Simulations are used to show that the simple mapping of the difference in transmitted/reflected IR energy between a saturated vortex-shaped beam and a Gaussian reference with a confocal microscope affords the generation of high-resolution vibrational absorption images. On the basis of experimentally relevant parameters, the simulations of the differential absorption scheme reveal a spatial resolution better than a tenth of the wavelength for incident energies about a decade above the saturation threshold. The saturated structured illumination concepts are thus expected to be compatible with the establishment of point-like point-spread functions for measuring the absorbance of samples with a scanning confocal microscope recording the differential transmission/reflection.

Figures (3)

(a) Differential absorption microscopy scheme where reference Gaussian (G) and vortex-shaped (V) pulses are focused by a lens (L) alternatively on the sample (SPL) and where the transmitted intensity is integrated by a detector (D). A scanner is used for producing images (xy). (b) Same as (a) but with confocal spatial filtering of the transmitted beam using a pinhole (PH). (c) Illustration of the transmission and change in population of a system of two-level oscillators for increasing incident intensity. (d) Difference Δ between vortex and reference PSF.